SAW (Surface Acoustic Wave) filters of the DMS (Dual Mode SAW filter) type are preferably used as RF filters when the intention is either to change from asymmetrical to symmetrical signal routing or to achieve small filter dimensions. DMS filters comprise at least two resonators which are arranged in the same acoustic track, acoustically couple to one another and are preferably alternately connected to the input and output sides of the RF filter. The number of input and output transducers laterally arranged beside one another in the longitudinal direction determines the electrical performance of the filter substantially via the respective transducer impedances and the resonances occurring between the individual transducers.
In order to improve the selectivity of DMS filters, it is known practice to connect the filters to individual resonators or to one or more basic elements of a ladder-type structure. In this case, a so-called basic element respectively comprises at least one acoustic resonator arranged in a series path and an acoustic resonator connected to ground in parallel therewith in a shunt branch.
On account of their symmetrical structure, DMS filters are also suitable for symmetrizing an asymmetrical or single-ended signal or converting it into a balanced signal. However, it is also possible to operate a DMS filter in an asymmetrical-asymmetrical or symmetrical-symmetrical manner.